The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material (or ink) is preferentially retained by image areas on a substrate. When a suitably prepared surface is moistened with water and an ink is applied, the background or non-image areas retain the water and repel the ink. The image areas then accept the ink and repel the water. Ink on the image areas can then be transferred to the surface of a suitable receiving material, such as cloth, paper or metal, thereby reproducing the image. Commonly, the ink is transferred to an intermediate material known as a blanket which in turn transfers the ink image to the surface of the final receiving material upon which the image is to be reproduced.
Conventional lithographic printing plates typically include a hardenable polymeric layer (usually visible or UV light-sensitive) on a suitable metallic or polymeric support. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are also known. They include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developers to provide the printable image.
Dry planography, or waterless printing, is well known in the art of lithographic offset printing and provides several advantages over conventional offset printing. Dry planography is particularly advantageous for short run and on-press applications. It simplifies press design by eliminating the fountain solution and aqueous delivery train. Carefuil ink water balance is unnecessary, thus reducing rollup time and material waste. Use of silicone rubber, [such as poly(dimethylsiloxane) and other derivatives of poly(siloxanes)] have long been recognized as preferred waterless-ink repelling materials.
In the lithographic art, materials that release or repel oil based inks are usually referred to as having "oleophobic" character. Herein, ink repelling materials are defined as "melanophobic" and, conversely, the term "melanophilic" is used to describe ink "loving" or accepting materials.
The planographic materials noted above are the object of considerable development effort in the industry, but due to a number of performance problems or costs, there remains a need to explore other means for providing printed images using sources of digital information, such as digitally controlled printing devices.
Many different types of digitally controlled imaging or printing systems are known. These systems utilize a variety of actuation mechanisms, marking materials and recording media. Examples of such systems include, but are not limited to, laser electrophotographic printers, LED electrophotographic printers, dot matrix impact printers, thermal paper printers, film recorders, thermal wax printers, dye diffusion thermal transfer printers, and ink jet printers. Due to various disadvantages or limitations, such digital printing systems have not significantly replaced mechanical printing presses and the more conventional printing plates described above, even though these older systems are labor intensive and inexpensive only when more than a few thousand copies of the same image are wanted. Yet, there is considerable activity in the industry to prepare and use recording media that can be digitally imaged and used to provide high quality, inexpensive copies in either a short- or long-run job.
Ink jet printing has become recognized as a viable alternative in the industry because of its non-impact deposition of ink droplets, low-noise characteristics, its use of plain paper as a receiving material, and its avoidance of toner transfer and fixing (as in electrophotography). Ink jet printing mechanisms can be characterized as either continuous ink jet or "drop on demand" ink jet printing. Various ink jet printers and systems are currently available for a number of markets, including their common use with personal computers. A very essential aspect of such systems, of course, is a printing ink that has all of the necessary properties for a given application.
Various teachings about ink jet printing including nozzles and drop modulation are described, for example, in U.S. Pat. No. 1,941,001 (Hamsell), U.S. Pat. No. 3,373,437 (Sweet et al), U.S. Pat. No. 3,416,153 (Hertz et al), U.S. Pat. No. 3,878,519 (Eaton), and U.S. Pat. No. 4,346,387 (Hertz).
Conventional continuous ink jet printing utilizes electrostatic charging "tunnels" that are placed close to the point where the ink drops are formed in a stream. In this manner, individual drops may be charged, and these drops may be deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes known as a "catcher") may be used to intercept the charged drops, while the uncharged drops are free to strike the recording medium. If there is no electric field present, or if the drop break off point is sufficiently far from the electric field (even if a portion of the stream before the drop break off point is in the presence of an electric field), then charging will not occur. In order to avoid these complications and uncertainties, it is desirable to avoid the use of electrostatic charging tunnels when using ink jet printing.
Printing plates have been made using ink jet printing and various techniques, as described for example in U.S. Pat. No. 4,003,312 (Gunther), U.S. Pat. No. 4,833,486 (Zerillo), U.S. Pat. No. 5,501,150 (Leenders et al), U.S. Pat. No. 4,303,924 (Young), U.S. Pat. No. 5,511,477 (Adler et al), U.S. Pat. No. 4,599,627 (Vollert), U.S. Pat. No. 5,466,658 (Harrison et al), and U.S. Pat. No. 5,495,803 (Gerber et al).
JP Kokai 53-015905 describes the preparation of a printing plate by ink jet printing using an ink comprising an alcohol-soluble resin in an organic solvent onto an aluminum support. Similarly, JP Kokai 56-105960 describes ink jet printing using an ink comprising a hardening substance, such as an epoxy-soybean oil, and benzoyl peroxide, or a photohardenable polyester, onto a metallic support. These inks are disadvantageous in that they include light-sensitive materials or environmentally unsuitable organic solvents.
EP-A-0776,763 (Hallman et al) describes ink jet printing of two reactive inks that combine to form a polymeric resin on a printing plate. JP Kokai 62-25081 describes the use of an oleophilic liquid as an ink jet ink.
Inks for high speed ink jet drop printers must have a number of special characteristics. Typically, water-based inks have been used because of their conductivity and viscosity range. Thus, for use in a jet drop printer the ink must be electrically conductive, having a resistivity below about 5000 ohm-cm and preferably below about 500 ohm-cm. For good fluidity through small orifices, the water-based inks generally have a viscosity in the range between 1 and 15 centiposes at 25.degree. C.
Beyond this, the inks must be stable over a long period of time, compatible with ink jet materials, free of microorganisms and functional after printing. Required functional characteristics include resistance to smearing after printing, fast drying on paper, and being waterproof when dried.
Thus, problems to be solved with ink jet inks include the large energy needed for drying, cockling of large printed areas on paper surfaces, ink sensitivity to rubbing, the need for an anti-microbial agent and clogging of the ink jet printer orifices from dried ink.
Some of these problems may be overcome by use of polar, conductive organic solvent based ink formulations. However, non-polar solvents generally lack sufficient conductivity. Addition of solvent soluble salts can make such solvents conductive, but such salts are often toxic, corrosive and unstable, and therefore present a number of reasons why they should be avoided.
It would be desirable to have a means for preparing printing plates using ink jet printing techniques in an economical fashion, at high speed without the limitations of requiring an electrically conductive ink and without the problems noted above. It is also desirable that printing plates prepared in this fashion would be long wearing, that is be useful for long press runs.